1. Field of The Invention
The present invention relates generally to liquid electrophotographic (or electrostatographic) developers having easy cleaning properties. In particular, the Present invention relates to nonaqueous dyed latex liquid developers that contain a selected amount of mineral oil which provides easy cleaning of equipment in which the developer is employed.
2. Brief Description of Prior Art
Liquid developer compositions are used in office copy machines, computer print-out devices, lithographic master Preparation and the like to create a visible counterpart from a latent electrostatic image. Liquid toners generally consist of five components: a carrier liquid, coloring agent, fixative agent, dispersing agent, and charge director. In any given developer composition, there may be one or more of each of these components. Also, one or more chemicals in such developer compositions may simultaneously have multiple functions. For example, a dispersing agent may also act as a fixative. Moreover, when a polymeric dispersing agent is employed, the combination of coloring agent, fixing agent, and dispersing agent is sometimes called a dyed latex toner polymer.
In the development of the electrostatic latent image to a visible image, the solid toner particles (including dyes or pigments) in the developer composition either migrate to the charged areas or the uncharged areas but not to both. If the toner particles go to the charged areas, this is called positive development. If the toner particles go to the uncharged areas, this is called reversal development. The coloring agent should be essentially insoluble in the carrier liquid and preferably contain no contaminants which are soluble therein. Dyes are selected for their solubility in the fixing agent and insolubility in the carrier liquid as well as for their color. Moreover, pigments are chosen on the basis of proper color, the best intrinsic surface or migration properties, ease of grinding to a desired fine particle size, and the differential between its specific gravity and that of the carrier liquid.
Dyes are usually employed for coloring latex toner. The resultant mixture is commonly called "a dyed latex". Dyes are incorporated therein by reacting them or by dissolving them into the latex polymer.
The fixative agent aids in the making of the toned or visual image a permanent part of the underlying substrate (e.g., paper). In a dyed latex, the fixative agent portion is generally a synthetic polymer or copolymer which has the desirable characteristics of chemical stability, an unobjectable color, and is insoluble in the liquid carrier as well as being compatible with the substrate onto which the image is deposited. There are many synthetic resins useful for this purpose.
The last component of a dyed latex liquid developer is the charge director. The charge director must be soluble or dispersible in the liquid carrier and must create or augment an electrostatic charge on micron or sub-micron toner particles. The patent literature is replete with different charge director compositions. Typical charge directors are metal salts of long chain fatty acids, both substituted and unsubstituted.
U.S. Pat. Nos. 3,753,760; 3,900,412; 3,990,980; and 3,991,266, all of which issued to Kosel and are each incorporated herein by reference in their entirety, teach the creation of a multifunctional amphipathic latex molecule which incorporates in one molecule the colorant agent, the dispersing agent, and the fixative agent. Thus, liquid latex developers as these are sometimes called, have only three components: the carrier liquid, the multifunctional latex particle ("a dyed latex"), and the charge director.
Mineral oil has been added to commercial latex-type and hybrid-type liquid developers as an optional dispersing ingredient. The amount of mineral oil added in such instances was generally less than about 0.25% by weight of the total developer formulation.
It has now been found that when mineral oil is added to a dyed latex-type or hybrid-type liquid toner in greater amounts, it acts as an antidrying agent; thus preventing the formation of a hard, dry toner film on internal machine surfaces. Therefore, machines which employ such mineral oil-containing liquid developer formulations remain cleaner and, when cleaning is desired, are easier to clean than the corresponding previously used liquid developers.